Search for exomoons turns up fluffy, Earth-sized planet

No moons found, but the lightest planet ever measured pops out of Kepler data.

In December, scientists announced the serendipitous observation of an event that may represent the first discovery of an exomoon—a moon that orbits a planet outside our solar system. Unfortunately, it's equally likely that the event was caused by a distant star orbited by a large planet. Since the event required a chance alignment of stars, we're unlikely to observe the object again, which means we're stuck in scientific limbo when it comes to exomoons.

But there's a team trying to change that. The Hunt for Exomoons with Kepler (HEK) project has been searching the observatory's data for signs of a distant moon. So far, they've come up blank, but in sifting through the Kepler data more carefully, they've provided a more detailed picture of the planets it contains. In a recently released publication, they describe one that's only a bit larger than Earth but only slightly more dense than water.

Kepler searches for exoplanets by watching for signs of them transiting between their host star and Earth, which causes a temporary dimming of the light of the star at regular intervals. From the degree of dimming, we can then infer the size of the planet. But that data normally doesn't tell us much about what the planet looks like; we need a separate measure of its mass so that we can know its density and infer its composition.

However, the HEK team realized that even the limited information obtained by Kepler could show signs of an exomoon. That information would come in two forms. One is what's called a Transit Duration Variation. If, during the exoplanet's transit, its moons are aligned so that they're either in front of or behind the planet (relative to the host star), the transit will look normal. During other orbits, the exomoons may be ahead of or behind their planet's orbit. In that case, if they're big enough, they'll also obscure some of the light from the host star, seemingly extending the time of the transit.

But that's not the only way an exomoon would affect transits. When the exomoon is ahead of the planet's orbit, it will provide a gravitational tug that hurries the planet along; when it's behind, it will tug to slow the exoplanet down. These events will cause subtle shifts in when the planet first appears in front of its host star, which will also be apparent in the Kepler data.

Moons aren't the only things that can do this. A lot of the exosolar systems we've discovered are packed densely with planets, which allows the other exoplanets to have similar effects, pulling their fellow planets forward and back as they orbit.

The authors turned the HEK analysis pipeline onto a set of eight objects identified by Kepler, all of them in orbit around M-dwarf stars (smaller, redder stars than our Sun). And the software did suggest that there was something interesting going on in two of the systems. But those systems both had at least two planets in them, raising the prospect that the variations were caused by their gravitational interactions.

The authors built a model of both exosolar systems and found that they could get gravitational interactions to produce the timing and duration variations they've observed. But to do so, the planets have to have a specific gravitational pull, which in turn implies a specific mass. So while finding that there was no clear evidence for exomoons, the HEK team ended up weighing four exoplanets in two different systems. Combined with the Kepler information on their size, it's possible to figure out how dense they are.

Two of them (KOI 314b and KOI 784.02) appeared to be what are termed super-Earths, slightly larger than our planet (radii 1.6 and 1.8 times that of Earth's) and roughly as dense. The other two (KOI 314c and KOI 784.01) were similar in size but far less dense. KOI 784.01 had a density half that of Earth's while KOI 314c was only slightly more dense than liquid water: 1.31 grams per cubic centimeter. For context, the Earth's density is 5.52 grams per cubic centimeter. Combined with its relatively small size, that makes KOI 314c the lightest planet we've ever discovered.

Although it's technically possible that the planet is little more than a giant ball of water, the HEK team favors an alternate explanation: it's an evaporating Neptune. The planet's density suggests that it has an atmosphere that's mostly hydrogen and helium, but a very thick one: approximately 17 percent of the planet's radius. However, the object's gravity would probably have trouble hanging on to that gas given how close it orbits to its host star every 23 days. The researchers argue that it started its existence as something closer to Neptune, but it has bled off some of its atmosphere since.

The paper is yet another indication that the Kepler data set is much more than a resource to discover exoplanets. Although the data is deceptively simple—how much light are we seeing from a given star—when analyzed in detail, it can provide remarkable information about distant worlds.